Treat `const` as a keyword instead of a type-hint

PHP does not use `const` as a type-hint like it does with `array`,
`callable`, and so forth.  Semantically we should treat it as a
built-in keyword instead.
Add `enddeclare` to the list of keywords

This list at

    http://php.net/manual/en/reserved.keywords.php

shows that this is a reserved keyword in PHP.
Treat `eval` as a built-in keyword
Show links to the official site and wiki when customizing PHP Mode

This patch adds two URLs to the information for the PHP Mode group so
that when users run `M-x customize-group` they will see links to
websites with additional information about PHP Mode.
Update php-mode-modified for changes made today